A phased array antenna is an array of antenna elements connected together that are switched between transmit and receive channels. Steering is accomplished by controlling the phase and amplitude of the elements. It is also necessary to adjust the phase and amplitude in order to correct or compensate for errors and inaccuracies due to environmental and other conditions. In order to make the desired adjustments, it is necessary to calibrate and tune the antenna system. The ability for a multi-element array antenna system to electronically form a beam in a predetermined direction is based on the accuracy of both the phase and amplitude settings at each individual element. Phased array antennas typically are comprised of thousands of elements and are able to electronically steer multi-beams throughout a prescribed sector to provide both search and targeting information that is usually integrated with other weapon systems.
Phased array systems have been passive in nature. The advantage that the passive type architecture has over the active type architecture is the ability to be calibrated once at the factory and be able to maintain this calibration over a very long period. This ability is due to the passive nature of many of the components within the beamforming network that provides the amplitude and phase levels at each of the elements. The next generation of ships will favor integrating active type systems that represent a higher degree of complexity then the passive type architecture. Due to the complex nature of these systems, active system calibration is necessary to maintain the ability to operate at the high level of performance necessary to carry out a mission.
Presently, these large antenna apertures are calibrated using a Near Field Scanner (NFS) system prior to placement into the ships super-structure. The NFS uses a small waveguide probe placed close proximity to the antenna aperture and is moved over the complete surface using a 2-axis scanner mechanism. As the probe is positioned in front of each element a small calibration signal is transmitted to the element and associated RF equipment behind the element. This enables a complete electrical characteristic (or calibration) to be performed from each array element to the receiver output. Unfortunately, the physical size and weight of these scanners and the associated mechanical support structure needed to perform this level of calibration makes a scanner type structure unmanageable to be used for in-situ type measurements
The ability to inject real time calibration signals into a phased array receive antenna allows the system to maintain a high level of operational performance. This is especially important when an array is being used in a multi-functional role, such as in the Navy's Advanced Multifunction RF Concept (AMRFC), as described in “Advanced Multifunction RF System,” P. Hughes, J. Choe, and J. Zolper, GOMAC Digest, 194-197 (2000). Previous and current array calibration schemes provide a mix of techniques that are used before and after installation into a platform.
In one approach, array calibration is performed using both internal and external signal injection, which include near or far field calibration techniques. These techniques record vast amounts of data that become part of a master look up table. This look up table provides corrections for both the amplitude and phase control settings for steering and amplitude weighting of the array. To accomplish the calibration, however, the array is removed or large moveable structures utilized that necessitate placing the system out-of-service while the calibration is performed. The array is therefore typically not recalibrated until it is removed from service when general maintenance is performed, therefore in the interim the system can be well out of calibration.
Another technique described in U.S. Pat. No. 5,559,519, incorporated herein by reference, involves calibrating an active phased array antenna using a test manifold coupled to the transmit output of a plurality of antenna modules. Although the system permits recalibration using a known far-field source, it cannot recalibrate antenna elements that are beyond the test manifold coupler.
Another calibration technique injects small calibration signals after the antenna element. In doing this any mutual coupling that occurs due to the element proximity to each other is not included in the calibration. In order to completely calibrate the array, the element “health” must be included in the calibration to accurately set the amplitude and phase settings. There are other calibration techniques that rely on the “unchanging” nature of the mutual coupling between the elements. These techniques, which provide a powerful calibration capability, become corrupt if the elements themselves become defective.
As array systems become more complex and advanced, the need to have available accurate and up-to-date calibration data becomes apparent. The introduction of advanced active arrays means that future systems will require more frequent calibration than passive arrays.